Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Polymeric membranes have proven to operate successfully in industrial gas separations such as in the separation of nitrogen from air and the separation of carbon dioxide from natural gas. Cellulose acetate (CA) commercial spiral wound and hollow fiber membranes have been used extensively for natural gas upgrading. However, CA membranes still need improvement in a number of properties including selectivity, performance durability, chemical stability, resistance to hydrocarbon contaminants, resistance to solvent swelling, and resistance to CO2 plasticization. Natural gas often contains substantial amounts of heavy hydrocarbons and water, either as an entrained liquid, or in vapor form, which may lead to condensation within membrane modules. The gas separation capabilities of CA membranes are affected by contact with liquids including water and aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (collectively known as BTEX). The presence of more than modest levels of liquid BTEX heavy hydrocarbons is potentially damaging to the CA membrane. Therefore, precautions must be taken to remove the entrained liquid water and heavy hydrocarbons upstream of the membrane separation steps using expensive membrane pretreatment system. Another issue of CA polymer membranes that still needs to be addressed for their use in gas separations in the presence of high concentration of condensable gas or vapor such as carbon dioxide (CO2) and propylene is the plasticization of the polymer by these condensable gases or vapors that leads to swelling of the membrane as well as a significant increase in the permeance of all components in the feed and a decrease in the selectivity of CA membranes. For example, the permeation behavior of CO2 in CA membranes is different when compared to some other glassy polymers in that above a certain pressure level, the permeability coefficient begins to increase with pressure due to the onset of plasticization by the CO2. A high concentration of sorbed CO2 leads to increased segmental motion, and, consequently, the transport rate of the penetrant is enhanced. The challenge of treating gas, such as natural gas, that contains relatively large amounts of CO2, such as more than about 50%, is particularly difficult.
In addition, some natural gas feed has high CO2/C2+ concentration (usually CO2>70%). Membranes can be used to recover the high value natural gas liquid while removing CO2 from natural gas. Membranes can separate CO2 from CH4 and C2+ and recover C2+ from the membrane retentate. When using membranes for this separation, the feed side temperature drops significantly due to CO2 permeation (J-T effect), and the feed gas dew point increases as CO2 permeates, therefore liquid comes out from membrane system. The membranes, however, show significantly decreased membrane permeance in the presence of liquid aliphatic hydrocarbons, liquid aromatics, or both liquid aliphatic hydrocarbons and liquid aromatics.
Therefore, new robust membranes with stable performance under repetitive short term exposure to liquid hydrocarbon condensation, high resistance to hydrocarbon contaminants, high resistance to solvent swelling, and high resistance to CO2 plasticization are desired for natural gas upgrading.
Polymeric membrane materials have been found to be of use in gas separations. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (see, for example, U.S. Pat. No. 6,932,589). The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are however several other obstacles to use a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate or flux. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Therefore, commercially available polymer membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. See U.S. Pat. No. 3,133,132. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. The thin dense skin layer and the porous non-selective support layer are formed simultaneously from the same polymeric material. Therefore, the cost of a new asymmetric integrally skinned polymeric membrane could be very high when an expensive new polymeric membrane material is used. One attempt at reducing the cost of the new membranes has been the development of thin film composite (TFC) membranes, comprising a thin selective skin layer of a high cost, high performance polymer deposited on a porous, low cost, non-selective support membrane. See, for example, “Thin-Film Composite Membrane for Single-Stage Seawater Desalination by Reverse Osmosis” by R. L. Riley et al., Applied Polymer Symposium No. 22, pages 255-267 (1973). TFC membranes can be formed from CA, polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. TFC hollow fiber membranes can be fabricated via a one-step co-extrusion process using two different polymeric spinning solutions and a triple-annular spinneret. For example, U.S. Pat. No. 8,337,598 disclosed a TFC hollow fiber membrane with a core cheap polymer layer and a sheath polyimide polymer layer. However, fabrication of TFC flat sheet membranes that are free from leaks is difficult, and normally fabrication requires multiple steps and so is generally more complex and costly. For example, TFC flat sheet membranes reported in the literature are typically formed by first fabricating a porous asymmetric integrally skinned polymeric membrane via a phase inversion technique followed by adding a thin selective layer on top of the porous asymmetric integrally skinned polymeric membrane by coating, laminating, interfacial polymerization, or other method.
Light olefins, such as propylene and ethylene, are produced as co-products from a variety of feedstocks in a number of different processes in the chemical, petrochemical, and petroleum refining industries. Various petrochemical streams contain olefins and other saturated hydrocarbons. Typically, these streams are from stream cracking units (ethylene production), catalytic cracking units (motor gasoline production), or the dehydrogenation of paraffins.
Currently, the separation of olefin and paraffin components is performed by cryogenic distillation, which is expensive and energy intensive due to the low relative volatilities of the components. Large capital expense and energy costs have created incentives for extensive research in this area of separations, and low energy-intensive membrane separations have been considered as an attractive alternative.
In principle, membrane-based technologies have the advantages of both low capital cost and high-energy efficiency compared to conventional separation methods for olefin/paraffin separations, such as propylene/propane and ethylene/ethane separations. Four main types of membranes have been reported for olefin/paraffin separations. These are facilitated transport membranes, polymer membranes, mixed matrix membranes, and inorganic membranes. Facilitated transport membranes, or ion exchange membranes, which sometimes use silver ions as a complexing agent, have very high olefin/paraffin separation selectivity. However, poor chemical stability, due to carrier poisoning or loss, high cost, and low flux, currently limit practical applications of facilitated transport membranes.
Separation of olefins from paraffins via conventional polymer membranes has not been commercially successful due to inadequate selectivities and permeabilities of the polymer membrane materials, as well as due to plasticization issues. Polymers that are more permeable are generally less selective than are less permeable polymers. A general trade-off has existed between permeability and selectivity (the so-called “polymer upper bound limit”) for all kinds of separations, including olefin/paraffin separations. In recent years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success in terms of improving the membrane selectivity.
More efforts have been undertaken to develop metal ion incorporated, high olefin/paraffin selectivity facilitated transport membranes. The high selectivity for olefin/paraffin separations is achieved by the incorporation of metal ions such as silver (I) or copper (I) cations into the solid nonporous polymer matrix layer on top of the highly porous membrane support layer (so-called “fixed site carrier facilitated transport membrane”) or directly into the pores of the highly porous support membrane (so-called “supported liquid facilitated transport membrane”) that results in the formation of a reversible metal cation complex with the pi bond of olefins, whereas no interaction occurs between the metal cations and the paraffins. Addition of water, plasticizer, or humidification of the olefin/paraffin feed streams to either the fixed site carrier facilitated transport membranes or the supported liquid facilitated transport membranes is usually required to obtain reasonable olefin permeances and high olefin/paraffin selectivities. The performance of fixed site carrier facilitated transport membranes is much more stable than that of the supported liquid facilitated transport membranes and the fixed site carrier facilitated transport membranes are less sensitive to the loss of metal cation carriers than the supported liquid facilitated transport membranes.
U.S. Pat. No. 5,670,051 (Pinnau et al.) disclosed a solid polymer electrolyte fixed site carrier facilitated transport membrane comprising silver tetrafluoroborate incorporated poly(ethylene oxide). U.S. Pat. No. 7,361,800 (Herrera et al.) disclosed a process for the separation of olefin/paraffin mixtures using a silver cation-cheated chitosan fixed site carrier facilitated transport membrane. U.S. Pat. No. 7,361,800 disclosed the coating of a layer of chitosan on the surface of a microporous support membrane, wherein the support membrane is made from polyesters, polyamides, polyimides, polyvinylidene fluoride, polyacrylonitrile, polysulfones or polycarbonates.
US 2015/0025293 A1 (Feiring et al.) disclosed a new facilitated transport membrane comprising silver (I) cation exchanged fluorinated copolymer synthesized from a perfluorinated cyclic or cyclizable monomer and a strong acid highly fluorinated vinylether compound.
The composite facilitated transport membranes disclosed in the literature comprise an ultrafiltration or microfiltration membrane as the support membrane. The use of a TFC flat sheet membrane comprising a high performance polymeric selective layer and a cheap, porous, non-selective support layer for the preparation of fixed site carrier facilitated transport membranes for olefin/paraffin separations has not been reported in the literature.